I personally dislike "cute" fun wording- apart from anything else, it often obscures the meaning and then everyone is confused and gets it wrong.
"Semi-formal" works very well.

I heartily agree with colleenoz. People want to be original, but too often their good intentions end up being cutesy-poo and/or confusing.

I think people worry too much about wording for "dress code" on invitations. People do like to have an idea how to dress, but just as cuteness can be confusing, actually so can attempts to be too specific, in the form of making up categories like "elegant casual" or "festive attire." People understand "black tie," "dressy," or "casual," and I suppose "dressy casual" (which I hate; to me, it means, "casual, but we think you are too much of a slob to know that 'casual' doesn't mean ripped jeans"), but I'd steer clear of anything else, and I probably wouldn't even bother with "casual" or "dressy."

You can just let the nature of the event (type, time, and location) and the tone of the invitation speak for themselves. For example, we just sent out invitations for a cocktail party honoring an engaged couple. We didn't say anything about dress. But the invitation is for "cocktails and hors d'oeuvres," it's a Saturday evening, and the invitation is kind of fancy with a formal-ish script font. I am sure everyone will get the idea to dress nicely, as for a nice restaurant on a summer evening: nice slacks or a dress for women; slacks and nice shirt, jacket and tie or not, for men. No one will show up in either an evening gown or shorts and tee shirt! (And if they did -- no tragedy.)

I'd send something that is bright and fun. Like a black and white polka dot or stripe. Or something in colors that are polka dot. I saw a cute invite recently that is nothing but circles. I think Perfect Impression or Paper Impressions has it out.

Then I would say

Please join us to celebrate the graduation of
childs name one
childs name two

Date
place
time

No gifts please